1. Technical Field
The present invention generally relates to signal processing, and particularly relates to reducing the peak-to-average ratio (PAR) of complex-valued signals, such as communication signals for transmission in a wireless communication network.
2. Background
Standards for many communication techniques like cellular, Wireless Local Area Network (WLAN), digital TV broadcast, Asymmetric Digital Subscriber Line (ADSL) etc. use signal modulation techniques based on both amplitude and phase modulation. In comparison to pure phase (or frequency) modulation, amplitude-modulated signals require linear amplification for accurate signal reproduction. Nonlinearity in the amplification of such signals introduces significant problems, such as increased adjacent channel interference (ACI) and increases of error-vector-magnitude (EVM) for the signal.
Linear amplification presents challenges, particularly in the cost and power limited environments typical in wireless communication applications. For example, accommodating larger signal amplitude variations in a linear transmitter generally causes reduced power efficiency and/or higher circuit cost and complexity. Unfortunately, the current trend is to use modulation schemes that exhibit very large amplitude variations. For example, the introduction of HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) within the 3GPP standard will significantly increase transmit signal amplitude variations. Additionally, many standards for WLAN, digital TV broadcast, ADSL, etc., are based on Orthogonal Frequency Division Multiplex (OFDM) modulation techniques that are known to have a very large amplitude variation.
One obvious but unsophisticated technique to reduce the amplitude variation is to clip signal peaks to a certain level but this comes at the cost of potentially dramatically increased ACI and EVM. As one alternative in the OFDM signal context, it has been proposed to reduce amplitude variation by directly altering the mapping of the data onto the sub-carriers, such that the overall amplitude variation is lowered. This proposal, however, imposes restrictions on the OFDM signal itself, e.g., by allocating a large fraction of the sub-carriers for reducing amplitude variations, or by introducing a specific coding scheme.
Other approaches adopt some form of peak cancellation, as opposed to brute-force clipping. However, heretofore such approaches have entailed undesirable signal processing complexity, or have provided insufficient PAR reduction performance.